1. Field of the Invention
The invention is a pressure sensitive valve that in different forms can be used to provide: an adjustable resistance to flow, adjustable positive or negative pressure relief, negative pressure isolation, isolation of pressure sensitive devices (e.g. pressure monitors) from blood and controlled recirculation between the inlet and outlet of pumps, particularly extracorporeal circulation.
2. Description of the Prior Art
The simplicity and availability of the standard roller pump have made it the choice for extracorporeal circulation. This pump is widely used in dialysis, routine cardiopulmonary bypass and long term pumping such as extracorporeal membrane oxygenators, (ECMO) and left and/or right heart bypass. The standard roller pump maintains a constant flow independent of clinically expected changes in inlet or outlet pressures. Thus, a decrease in blood supply at the pump inlet, without a concomitant decrease in pump speed, can cause excessive suction leading to air embolism, thrombosis and damage by the "venous" cannula to the vessel's intima. When a roller pump is used with a bubble oxygenator, a decrease in venous flow that could result in the oxygenator blood level decreasing and in the worst case, air being pumped into the patient. The combination of constant flow and an arterial line that is accidentally clamped or kinked, or an arterial cannula is positioned against the vessel intima, can generate excessive pressures at the outlet of the pump which at the extreme, can blow up a connector, tube or an oxygenator. In addition, the new membrane oxygenators (the "lung" part of the Heart-Lung machine) are made of microporous membranes that require a positive blood to gas pressure to prevent gas bubbles from entering the blood through the pores. If a negative pressure is applied to the blood side, air can be drawn into the blood stream and pumped to the patient.
To overcome these potential dangers, bubble oxygenators have blood level detectors which stop the pump when the blood level drops below a preset level, or a floating ball valve (as for example made by Health Dyne Cardiovascular Inc. Costa Mesa Calif. 92626) that closes when the blood level drops. In closed systems such as ECMO or dialysis, collapsible bladders have been placed at the inlet to the pump such that at too high a suction, the bladder collapses actuating a microswitch which stops the pump. The pump restarts when the bladder refills. Others have designed roller pumps with a servomotor and a microprocessor control system. When the roller pump is used during cardiac surgery for venting the left ventricle or for returning shed blood from the chest cavity it requires constant surveillance by a trained perfusionist to assure that no excess suction is applied. Present solutions to protect against extreme inlet or outlet pressures that may be generated by the roller pump require either on-off control with the standard roller pump, extensive and expensive modifications to the standard pump or a watchful perfusionist. U.S. Pat. Nos. 4,515,589 and 4,767,289 (manufactured by Sarns/3 M Corp. as the "Safety Loop"), and 4,650,471, describe devices to be used with the roller pump that prevent too much suction. The former provides no adjustment over the pressure about which flow is controlled. The latter has adjustments capabilities for the inlet, but neither one provides relief for overpressurization at the outlet of the pump. Another solution is to use a centrifugal pump such as that made by Biomedicus of Minneapolis Minn. Its flow characteristics permit a limited negative pressure and outlet pressure to be generated, its costs however are high, about $10,000 for the pump module and $200 for each of the disposable pump heads.
During CPB it is also necessary to be able to control the venous blood flow to the venous reservoir (e.g. lower flow is required to go on partial bypass). This is usually done by a) raising the venous reservoir relative to the patient, b) partially clamping the venous line either manually with a tubing clamp or c) with a mechanically actuated device that clamps the tubing. Of these methods (b) is the most popular because it is simple and does not require additional hardware. However, it is not precise and requires the perfusionist to reach over to the venous line that may not always be very accessible.
It is obvious from the above that it would be of great clinical advantage to be able to provide control over the maximum pressure in an extracorporeal circuit and the maximum suction the patient is exposed to, to prevent gas entering the blood stream in a microporous membrane oxygenator, to be able to adjust precisely the resistance to blood flow in a tube as well as provide means that provide the standard roller pump with the advantages of a centrifugal pump without its associated high costs. These can be done with pressure sensitive valves and appropriate control devices.
Prior art pressure sensitive valves are made of a sleeve sealed in a housing with means to pressurized the interluminal space (the space between the housing and the sleeve). Pressure applied to the interluminal space acts upon the wall of the sleeve forcing the opposite walls of the sleeve to meet and close shut. This external force on the wall is counteracted by the pressure within the lumen of the sleeve and the elastic force of the deformed walls which tend to keep the walls apart. It is the net force of these two vectors that determines whether the sleeve is opened, closed or in between.
In industry, these valves are used as ON/OFF valves or as adjustable resistors known as pinch valves. Pinch valves are also used to adjust the resistance to flow using an external roller that pinches and thus controlling the degree of closure of the sleeve. If the wall of the sleeve is made sufficiently thin, the valve can also be used to transfer the pressure of the fluid within the sleeve to the interluminal space without significant changes in the transduced pressure. Thus, these devices can transmit the pressure of a fluid that may be corrosive to a pressure gauge while isolating the pressure gauge from the fluid.
In the medical field such valves, known as Starling resistors, are composed of a thin walled sleeve and require negligible transwall pressure difference to close them. They have been suggested for use to maintain or adjust pressure, (Robert Rushmore: Control of Cardiac Output, in Physiology and Biophysics 19.sup.th edition Ruch TC and Patton HD editors, WB Saunders Co. Phil. 1965)
U.S. Pat. No. 4,767,289 teaches that a Starling valve may be made of a thin wall tubing, both ends of which are sealed to a rigid connector which in turn are sealed to the housing providing flow through chamber. U.S. Pat. No. 4,515,589 teaches that the walls of the thin wall tubing may extend beyond the housing, be folded upon themselves and sealed over the external wall of the housing. Another manufacturing technique suggests that the inner wall of a resilient sleeve be affixed to the outer wall of the thin walled tubing and the outer wall of that sleeve be affixed to the housing. These techniques have one or more of the following disadvantages: 1) the thin wall tube is stressed over the edges of the housing, 2) the assembly requires sealing the thin wall tube to the connectors, 3) the discontinuities of the valve at the connection sight between the thin wall and the thick wall tubing can create turbulence and trapped vortices, a leading cause of thrombus generation, 4) the assembly is labor intensive and require multiple parts and 5) control over the interluminal pressure with present systems is provided by a cumbersome and bulky combination requiring a compliance chamber, a pressure manometer and interconnecting tubing.
These disadvantages may be the reasons for the lack of pressure sensitive valve available for clinical use. U.S. Pat. No. 4,250,872 by Tamari illustrates a valve made of unitary tubing a portion of which has been expanded and thinned walled to allow easy contraction by external fluid pressure. This valve however was not made to fully close (as illustrated in FIG. 5) nor was it preformed to close completely shut. The only pressure sensitive valve that is known to be used clinically in the extracorporeal circuit is the one incorporated at the inlet to the "Safety Loop" mentioned above. Its assembly is labor intensive and requires multiple parts. In addition, its housing is exposed to atmosphere and provides no mechanism to adjust the interluminal pressure. Senko Medical Instrument Mfg. Co., LTD. of Tokyo Japan manufactures a pressure relief valve intended for dialysis. It is made by interposing a thin wall a diaphragm made of a plastic sheet between the wall with the pressure port and the blood path. This method though very adaptable to mass production, results in a diaphragm that often is accidentally heat-sealed to the housing wall, preventing its free motion, thereby rendering it nonfunctional. It also has discontinuities at the connection sight between the thin wall and the thick wall tubing creating the areas of stagnation which are prone to thrombus formation.